DESCRIPTIVE MODEL OF THORIUM-RARE-EARTH VEINS
By Mortimer H. Staatz
SYNONYM: Rare-earth-thorium veins.
DESCRIPTION: Various thorium and rare-earth minerals in a quartz-potassium
feldspar-iron-oxide gangue in veins 1 to about 1,330 m long and less than 1 cm to about 16 m
TYPICAL DEPOSITS: Last Chance vein, Lemhi Pass district, Montana (Staatz, 1979); Little Johnnie vein, Powderhorn district, Colorado (Olson and Wallace, 1956); vein no. 12, southern Bear Lodge Mountains, Wyoming (Staatz, 1983); Wet Mountains area, Colorado (Armbrustmacher, 1988).
RELATIVE IMPORTANCE: A future thorium resource. Highest grade thorium resource in the United
States, second largest total resource of thorium (Staatz and others, 1979). Rare earths important byproduct in some deposits; in others, the principal
COMMODITIES: Th, rare earths (mainly light rare earths, but at Laughlin Peak, New Mexico,
the heavy rare earths most important).
OTHER COMMODITIES: None.
ASSOCIATED DEPOSIT TYPES (*suspected to be genetically related): Disseminated rare-earth minerals in both massive carbonatites and carbonatite
dikes; example: one of the world's largest rare-earth deposits in a massive
carbonatite at Mountain Pass, California (Olson and others, 1954).
REGIONAL GEOLOGIC ATTRIBUTES
TECTONOSTRATIGRAPHIC SETTING: Commonly associated with diverse suites of alkaline rocks and carbonatites.
Thorium-rare-earth veins generally occur in an outer ring around alkaline rocks
(fig. 1). May be as far as 16 km beyond outer limits of the alkaline rocks.
Veins most common in the eastern part of the Cordilleran belt associated with
continental crustal rocks (Staatz and Armbrustmacher, 1982).
REGIONAL DEPOSITIONAL ENVIRONMENT: Veins formed along fractures in brittle rocks. Vein fluids commonly traveled
many kilometers before deposition. In a few areas, such as the Powderhorn
district (Olson and Hedlund, 1981), all related igneous rocks are exposed. From the center, igneous alkaline
rock complex surrounds a massive carbonatite and is bordered by fenite.
Carbonatite dikes intrude outer part of alkaline rocks and neighboring country rock.
Thorium-rare-earth veins intruded into an outer zone (fig. 1).
Figure 1. Idealized model showing relationship of thorium-rare-earth veins to alkalic
rocks and carbonatites.
Cross section view.
AGE RANGE: Host rock for veins: mainly Precambrian, but in several areas is Cretaceous
and Tertiary. Veins: in Powderhorn and Wet Mountain districts, Colorado, formed
between very late Precambrian to Ordovician (Olson and others, 1977); in Lemhi Pass district, Idaho and Montana (Staatz, 1972), Bear Lodge Mountains, Wyoming (Staatz, 1983), and Laughlin Peak area, New Mexico (Staatz, 1985), formed in Tertiary.
LOCAL GEOLOGIC ATTRIBUTES
HOST ROCKS: Hard brittle rocks. Rocks include Precambrian quartzite, hornblende schist,
gneiss, granite; Upper Cretaceous Dakota Sandstone; Tertiary trachyte,
phonolite, and intrusive breccia.
ASSOCIATED ROCKS: Alkalic rocks, carbonatites, fenites.
ORE MINERALOGY: principal ore minerals in most deposits: thorite+monazite. Associated minerals: +brockite+allanite+bastnaesite. Exceptions: (1) Bear Lodge Mountains, Wyoming, no thorite,
principally monazite+brockite+bastnaesite; (2) Laughlin Peak area, New Mexico, neither thorite nor monazite,
principally either (a) brockite + xenotime or (b) thorium- and
GANGUE MINERALS: Principal minerals: quartz+iron oxides (goethite and (or) hematite)+potassium feldspar. Minor minerals: +barite+apatite+magnetite +rutile+anatase+zircon (Staatz, 1974).
STRUCTURE and ZONING: Veins usually fine grained and commonly heavily stained with iron oxides+manganese oxides. Mineral zoning unknown.
ORE CONTROLS: Large alkaline rock body or bodies, whose magma was source of vein fluids
within about 20 km of veins (Staatz, 1974). Joints and small faults that served both as conduits for ore fluids and as
sites of deposition.
ISOTOPIC SIGNATURES: Unknown.
FLUID INCLUSIONS: Unknown.
STRUCTURAL SETTING: All ore in tabular veins.
ORE DEPOSIT GEOMETRY: Veins of potential economic interest range in length from about 60 to about
1,330 m and in thickness from about 0.3 to about 16 m. Veins may strike in
almost any direction. Dips of all veins steep.
ALTERATION: Iron minerals, where present, altered to goethite+lepidocrocite+hematite. Clay minerals not common; thorite often metamict, sometimes narrow
zone of fenitization around vein.
EFFECT OF WEATHERING: Probably aided in forming iron-oxide minerals.
EFFECT OF METAMORPHISM: Not applicable.
GEOCHEMICAL SIGNATURE(S): Some enrichment of Th and rare earths in alkaline igneous rocks. Th tends to
disperse rapidly in stream sediments short distances below veins (Staatz and others, 1971). Heavy metals in stream sediments not diagnostic.
GEOPHYSICAL SIGNATURE(S): Radiation due to thorium used to locate most veins. Generally located by
hand-held geiger counter or scintillometer. Most veins too narrow and (or) poorly
exposed to locate with airborne radiation counters.
OTHER EXPLORATION GUIDES: Unknown.
OVERBURDEN: Most known veins have some part exposed at surface. Veins have been traced
from original exposure under as much as 10 m of overburden.